Endothermic blowing agents for surface migration of components in foamed products, compositions and applications

ABSTRACT

Compositions for producing foamed products which include additives and blowing agents which cause the surface treatment additives to migrate to the surfaces of foamed products. The products produced from the compositions require less additives because the incorporated additives are concentrated at the surfaces thereof.

This application is a divisional, application of application Ser. No.07/713,224, filed Jun. 11, 1991, now U.S. Pat. No. 5,166,222, which is adivisional application of application Ser. No. 07/467,585, filed Jan.19, 1990, now U.S. Pat. No. 5,045,585, which is a continuation-in-partapplication of application Ser. No. 07/352,307, filed May 16, 1989 nowU.S. Pat. No. 5,009,810.

TECHNICAL FIELD

The present invention relates to endothermic blowing agents for use inmaking foamed products such as cellular structured plastics or rubbers.In particular, the present invention relates to endothermic blowingagents which are used in combination with additives so as to enhancemigration of the additives to surfaces of foamed products during theirformation.

BACKGROUND ART

The term "foaming agent" or "blowing agent" is used to describe anysubstance which alone or in combination with other substances is capableof producing a cellular structure in a plastic or rubber mass. Thus, theterm includes gases which expand when pressure is released, solublesolids that leave pores when leached out, liquids which develop cellswhen they change to gases, and chemical agents that decompose or reactunder the influence of heat to form a gas.

Known liquid foaming agents include certain aliphatic and halogenatedhydrocarbons, low boiling alcohols, ethers, ketones" and aromatichydrocarbons. Chemical foaming agents range from simple salts such asammonium or sodium bicarbonate to complex nitrogen releasing agents, ofwhich azobisformamide is an important example.

Foaming agents, also known as chemical blowing agents (CBA's) can beutilized in all conventional plastics processes, such as extrusion,calendering, injection molding, coating, expansion casting, androtational molding.

Recognized advantages of foamed plastics include reduction in density,savings in material costs, improved electrical and thermal insulativeproperties, increased strength to weight ratio, and the elimination ofshrinkage, warpage, and sink marks in injection molded parts. Foamedplastic products include such diverse items as vinyl flooring, insulatedfood containers, structural foam furniture, business machine housings,simulated leather, and foamed core pipe.

Foaming agents are generally classified as physical or chemical.Chemical foaming agents (generally solids) undergo a chemicaltransformation when producing gas, while physical foaming agents undergoa generally reversible physical change of state. e.g., vaporization.

The two major categories of physical foaming agents include both liquidsand gases. The gas most often is compressed nitrogen. In injectionmolding processes which utilize physical foaming agents, the gas isinjected under )Ugh pressure directly into the polymer duringplastication, and the mixed polymer and gas are metered into the mold.When the pressure is relieved, the gas becomes less soluble in thepolymer and expands, forming the cellular structure. Nucleating agents,in the form of finely divided powders and chemical foaming agents,sometimes are used with the gas to yield a finer cell structure.

The preference for nitrogen is due to the fact that nitrogen is inert,nonflammable, leaves no residue in the polymer, and is not temperaturerestrictive. However, the use of processes involving nitrogen is limitedin the industry due to the requirement of licensed specializedequipment. Moreover, the process tends to produce foams with poorerperformance and appearance than those produced by the use of exothermicchemical blowing compounds.

Liquid physical foaming agents include volatile liquids which producegas through vaporization. Common liquid physical foaming agentsgenerally include short-chain aliphatic hydrocarbons (C₅ to C₇) andtheir chlorinated and fluorinated analogs. Liquid physical foamingagents may be used over a wide temperature range in low pressure andatmospheric processes, and are widely used to produce low densitythermoplastics, such as foamed polystyrene, and thermoset polymers, suchas polyesters, epoxy, and polyurethane foam systems.

Chemical foaming agents commonly referred to as blowing agents aregenerally solids that liberate gas(es) by means of a chemical reactionor decomposition when heated. They are necessarily selected for specificapplications or processes based on their decomposition temperatures. Inthis regard, it is important to match the decomposition temperature withthe processing temperature of the polymer to be foamed. If the polymerprocesses at temperatures below that of the chemical foaming agent,little or no foaming will occur. If the process temperature issignificantly above the foaming agent's decomposition temperature, poor(overblown, ruptured) cell structure and surface skin quality willlikely result.

Chemical foaming or blowing agents may be either inorganic or organic.The most common inorganic foaming agent is sodium bicarbonate. Sodiumbicarbonate is inexpensive, nonflammable and begins to decompose at alow temperature; however, it is used only to a very limited extent inthermoplastics. Differential thermal analysis has shown that sodiumbicarbonate decomposes over a broad temperature range and this range isendothermic, contributes to an open cell structure in the finishedproduct, and the released gas (carbon dioxide) diffuses through thepolymer at a much greater rate than nitrogen gas.

Presently used endothermic chemical foaming or blowing agents are mostlymixtures of sodium bicarbonate and sodium hydrogen citrate. The citrateis incorporated together with the sodium bicarbonate in order tofacilitate a complete acid assisted decomposition reaction to producecarbon dioxide gas. The mixture is also available in various polymerconcentrates at various loadings. The mixture is also available as ahydrophobized acid and carbonate which is a free non-dusting powder.

The major advantages associated with utilizing endothermic foaming orblowing agents over their exothermic counterparts include shortdegassing cycles, small cells, smooth surfaces, weight reductions,reduced cycle times, foamed products which have promptly paintablesurfaces, the foaming process is odorless, and the components of thefoaming agents are generally regarded as environmentally safe.

The major disadvantage of existing acid/carbonate systems involves theformation of corrosion on the process equipment. This corrosion isattributed to the action of the citric acid and sodium bicarbonate onthe lesser grades of steel used in some equipment. Another disadvantageassociated with existing acid/carbonate blowing agents is prematurereaction with water or moisture of the blowing agents when they areassociated with polymeric reaction mixtures. This premature reactionwhen occurring prior to a foaming process detrimentally effects thefinal products.

Organic foaming or blowing agents can be utilized in most polymerapplications and processes. These compounds release gas (usuallynitrogen and/or ammonia) over a narrow temperature range. The rate ofgas evolution for a given chemical foaming or blowing agent isdetermined by a temperature and time relationship. Applications forchemical foaming agents are generally divided into three areas: low,medium and high temperature processing polymers. There are numerousorganic foaming agents available that decompose at various temperatures.

Blowing or foaming agents are utilized in the initial process of formingfoamed products. Conventional foaming processes include extrusion,calendering, injection molding, coating, expansion casting, rotationalmolding and the like. Often, after initial formation, foamed productsare subjected to machining, surface treating and other various methodsto achieve a final product.

Particular post-formation processes used in the fabrication of foamedproducts include surface treatment processes which produce barriersurfaces or layers which are flame retardatant, chemically inert,provide structural or mechanical strengths, and the like. Such surfacetreatments have heretofore been limited to post-formation process stepswherein foamed products are treated with various reactants, such asreactive gases which chemically modify the exposed surfaces.

The present invention is an improvement over the prior known chemical,endothermic foaming or blowing agents which provide a highly stableendothermic blowing agent that does not prematurely react with water ormoisture and which reacts in a controlled fashion. These endothermicblowing agents are used in combination with additives to cause migrationof the additives toward the surfaces of foamed products as they areformed. This migration of surface treatment additives during theformation of the foamed products eliminates the need for conventionalpost-formation surface treatment processes.

DISCLOSURE OF THE INVENTION

It is accordingly one object of the present invention to provide a novelendothermic blowing agent which has an enhanced stability, releases gasin a controlled manner, and is resistant to moisture absorption.

A further object of the present invention is to provide an endothermicblowing agent Which does not contribute to the corrosion of processingequipment.

Another object of the present invention is to provide a combination of ablowing agent and additives or compounds which provide for surfacetreatment of foamed products during their formation.

Another object of the present invention is to provide for a polymericconcentrate for producing foamed products which incorporates the presentendothermic blowing agents and additives or compounds.

A still further object of the present invention is to provide a methodfor producing foamed products utilizing the above endothermic blowingagents.

An even further object of the present invention is to provide for foamedproducts which have been produced by a process that utilizes the presentblowing agents in combination with additives or compounds.

According to the above objectives, there is provided by the presentinvention endothermic blowing agents which include a polycarboxylic acidand an inorganic carbonate, each of which has been surface treated witha compound which prevents premature reaction due to water absorption.These blowing agents are used in conjunction with additives to cause theadditives to migrate to the surfaces of foamed products.

Also provided by the present invention is a polymeric concentrate thatincludes the present endothermic blowing agents, additives, and variouspolymers.

The present invention further provides for a method to produce the aboveendothermic blowing agents and for foamed products that are produced byprocesses that utilize the endothermic blowing agents and additives.

BEST MODE FOR CARRYING OUT THE INVENTION

The powdered endothermic blowing agents of the present invention areproduced by blending a surface treated polycarboxylic acid and a surfacetreated inorganic carbonate. The surf ace treatment of these materialscan be performed in a high intensity mixer at a temperature range ofbetween about 72° C. and about 135° C., with a preferred temperaturebeing about 80° C. The preferred level of surface treatment is fromabout 0.1% to about 10%, and most preferably about 3% for thepolycarboxylic acid and about 3% for the inorganic carbonate. Theformation of the surface treatment within this preferred range has beenfound to be effective in avoiding premature reaction resulting fromwater absorption.

In surface treating the polycarboxylic acid, a typical laboratory batchis prepared by adding about 1500 grams of polycarboxylic acid, e.g.,anhydrous citric acid to a 10 liter henschel mixer or other suitablehigh intensity mixer. The polycarboxylic acid is mixed at about 1500 rpmand heated until a temperature of about 80° C. is reached. Any heatingmeans such as hot water flowing through a water jacket surrounding themixer may be used to attain the desired temperature. After reaching thedesired temperature about 45.0 grams of a surface treating compound suchas stearates, oleates, glyceryl monostearate and other suitablehydrophobic treatment compounds is added to the heated polycarboxylicacid and mixing is continued at about 1500 rpm and about 80° C. for anadditional period of time of approximately five minutes. After fiveminutes the mixing speed is reduced to about 900 rpm and the mixture isallowed to cool below about 60°. Thereafter the surface treatment of thepolycarboxylic acid is complete and the mixture is unloaded.

In surface treating the inorganic carbonate, about 3500 grams of aninorganic carbonate, e.g., dry sodium aluminum hydroxy carbonate, ischarged into a 10 liter Henschel mixer. The inorganic carbonate is mixedat a speed of about 1500 rpm and heated by a suitable means such as ahot water jacket until a temperature of about 80° C. is reached. Afterreaching the proper temperature about 108.2 grams of a surface treatingcompound such as glyceryl monostearate, is added to the inorganiccarbonate and mixing is continued at about 1500 rpm for about fiveminutes while the temperature is maintained at about 80° C. After therequired mixing, the mixing speed is reduced to about 900 rpm and themixture is allowed to cool below about 60° C. Thereafter, the inorganiccarbonate has been sufficiently surface treated and may be unloaded fromthe mixer.

In preparing a large scale batch of the above surface treatedcomponents, about 200 pounds of polycarboxylic acid, e.g. citric acid,was charged into a 200 liter Henschel mixer. The polycarboxylic acid wasmixed until a temperature of about 68° C. was reached. In this caseneither steam nor hot water was needed to raise the temperature. Whenthe polycarboxylic acid reached a temperature of about 680° C. sixpounds of a suitable surface treating component, e.g., glycerylmonostearate, was added to the mixer. Mixing continued until thetemperature reached about 72° C. after which mixing was continued for anadditional period of time of about 5 minutes. Thereafter the mixture wasallowed to cool to below about 60° C. and was unloaded from the mixer.The batch surface treatment procedure was repeated for the inorganiccarbonate using about 150 pounds of dry inorganic carbonate, e.g.,sodium aluminum hydroxy carbonate, and about 4.5 pounds of the surfacetreating component, e.g., glyceryl monostearate.

The present endothermic blowing agents are prepared by blending togetherthe surface treated polycarboxylic acid and the surf ace treatedinorganic carbonate. In a preferred embodiment about 1029 grams ofsurface treated inorganic carbonate, e.g., sodium aluminum hydroxycarbonate, was placed into a 10 liter Henschel mixer together with about1471 grams of surface treated polycarboxylic acid, e.g., citric acid.The mixture was mixed at about 1500 rpm for a period of time of aboutfive minutes. Thereafter the mixing speed was reduced to about 900 rpmfor a short period of time and unloaded from the mixture.

In the above discussion, the preferred polycarboxylic acid used wascitric acid. However, for purposes of the present invention, othersuitable carboxylic acids include those of the formula: HOOC--R--COOHwhere R is an alkylene group of 1 to about 8 carbon atoms which may alsobe substituted by one or more hydroxy groups or keto groups and may alsocontain unsaturation. Also included are salts and half salts. Preferredpolycarboxylic acids include citric acid, fumaric acid, tartaric acid,sodium hydrogen citrate and disodium citrate.

The preferred inorganic carbonate utilized in the invention is sodiumaluminum hydroxy carbonate. However, acceptable results are alsoachieved by also using sodium bicarbonate as well as alkali and alkalineearth metal carbonates and carbonates generally.

In the above examples glyceryl monostearate was utilized as thepreferred surface treatment compound. However, surface treatment can becarried out utilizing any reagent which renders the polycarboxylic acidand the inorganic carbonate hydrophobic. In this regard, suitablesurface treating components found useful for urposes of the presentinvention included various monoglycerides, diglycerides, stearic acid,fatty acids and salts, titanates, zirconates and silane coupling agentsin addition to the above glyceryl monostearate.

In combining the surface treated polycarboxylic acid and the surfacetreated inorganic carbonate to produce the blowing agent the weightratio of these two components needs to be determined based on thedesired structure and composition of the final foam product to beproduced. Typically, the weight ratio of surface treated polycarboxylicacid to surface treated inorganic carbonate may vary from about 0.25 toabout 10.00. However, the weight ratio is prefarably maintained betweenabout 0.5 and about 1.5.

The endothermic blowing agent is preferably used by incorporating itinto a polymeric concentrate which is used to produce foam products. Inone embodiment according to the present invention the polymeric orplastic concentrates were prepared at loading levels of from about 5% toabout 50% in a single or twin screw extruder utilizing normal techniquesknown to those schooled in the art of plastic compounding. The extrudermust, however, be operated below about 149° C. in order to keep theendothermic blowing agent from foaming prematurely.

In these polymeric or plastic concentrates citric acid may be replacedby other polycarboxylic acids such as fumaric acid, tartaric acid andsodium hydrogen citrate and other acid salts. The novel interactionbetween the polycarboxylic acid and the inorganic carbonate controls thecomposition and the volume of the above gases which serve to foam aplastic for foamed products.

The application areas for these endothermic products include polyvinylchloride, polyolefins, engineering resins, polystyrene, polyphenyleneoxide and other polymeric systems. These foamed products displayexcellent weight reduction and drop weight impact properties and areoutlined in the following examples given by way of illustration only.

The blowing agents of the present invention have been found tounexpectedly cause migration or flotation of various platelet fillers,flame retardants, and polar resin systems to the inner and outersurfaces of foamed products when used in combination with the blowingagents, thereby forming barrier surfaces like the foamed articles.

In particular, it has been discovered that platelet fillers, such asmica or talc, can be made to migrate to the inner and outer surf aces ofpolymers. The platelet filler migrates under the pressure and dilutiongas laws of an endothermic chemical blowing agent in fabricated partssuch as pipe, blow molded containers, and 1-10 mil film. Pipe extrusionstudies with mica at 5-50% levels in combination with the above blowingagents at 0.1 to 2.0% active levels have exhibited mica migration toboth the inner and outer surface of 1" diameter pipe samples. Controlsamples without the endothermic blowing agents failed to migrate themica particles to the surface. Similar test results with mica andendothermic-blowing agents in 4 mil film has shown this mica migrationto the resin surfaces.

Platelet flame retardants such as precipitated alumina trihydrate,magnesium hydroxide, magnesium carbonate, hydrotalcite, calcium aluminumhydroxide, and delaminated clays with an aspect ratio of at least 2:1have also been made to migrate to the surface of thermoplastic resinswith the addition of the above blowing agents. By migrating the flameretardant to the surface of the polymer, the loadings required toachieve flame retardant properties are reduced.

In further expanding this barrier technology, highly polar resins suchas ACN, PVA, EVOH, and DuPont Selar RB have been made to migrate by theuse of the above blowing agents. These polar resins (at 5-50% loadings)enhance the barrier properties of blow molded containers or injectionmolded parts toward gasoline and aggressive chemical environments.

The blowing agent migrates the polar resin system to both surfaces ofthe blow molded container. This method creates a novel barrier surfacewhich will resist and meet current automotive fuel permeationrequirements. The novel barrier technology is simple, cost effective,and requires no major machine modifications. In addition to automotivefuel tanks, the biggest potential for growth of this technology is inthe area of industrial, household and agricultural chemical containers.

As described in detail above, the present invention involves endothermicblowing agents which are mixtures of polycarboxylic acids and inorganiccarbonates, wherein the polycarboxylic acids and the inorganiccarbonates each have been surface treated with a component that preventswater from being absorbed thereby. This endothermic blowing agents areused in combination with additives or compounds and cause migration ofthese additives to the surfaces of foamed products during theirformation.

The level of surface treatment of the polycarboxylic acid and theinorganic carbonate is between about 1.0 and 5.0 percent by weight. In apreferred embodiment the level of surface treatment of saidpolycarboxylic acid is about 3.0 percent by weight and the level ofsurface treatment of said inorganic carbonate is about 3.0 percent byweight.

The ratio of the polycarboxylic acid and the inorganic carbonate mayvary over a wide range and will be determined by the final desiredproducts. Generally, the weight ratio of the surface treatedpolycarboxylic acid to the surface treated inorganic carbonate isbetween about 0.25 and about 10.00, with a more desired weight ratiobeing between about 0.50 and about 1.50.

As further discussed in detail above, the present invention alsoinvolves a method of making the endothermic blowing agent. Generally,this method involves the steps of:

(a) heating a dry powder of polycarboxylic acid to a temperature ofbetween about 68° C. and about 80° C. and adding thereto a surfacetreating component to form a surface treated polycarboxylic acid;

(b) heating a dry powder of an Inorganic carbonate to a temperature ofbetween about 68° C. and about 80° C. and adding thereto a surfacetreating agent to form a surface treated inorganic carbonate and;

(c) mixing together the surface treated polycarboxylic acid and thesurface treated inorganic carbonate to form the endothermic blowingagent.

In this method, each of the steps is performed in a high intensity mixerin which the components are agitated.

The endothermic blowing agent is ideally incorporated into a polymericconcentrate that is used for producing foamed products. The polymericconcentrate includes both the endothermic blowing agent, a polymerreaction mixture and one or more additives or compounds. Particularpolymer reaction mixtures are those that may be used to produce polymersof polyvinyl chloride, polyolefins, engineering resins, polypropyleneand polystyrene.

Finally, the present invention involves foamed polymeric products thathave been produced by both the above blowing agents and the abovediscussed polymeric concentrates. Such products may be made by processesthat involve either extrusion, calendering, injection molding, coating,expansion casting or rotational molding. These products have sufficientconcentration of the additives or compounds to render them sufficientlyflame retardant, chemically inert or mechanically strong, depending onthe particular additive incorporated.

Although the invention has been described with reference to particularmeans, materials and embodiments, from the foregoing description, oneskilled in the art can ascertain the essential characteristics of thepresent invention and various changes and modifications may be made toadapt the various uses and characteristics thereof without departingfrom the spirit and scope of the present invention as described in theclaims that follow.

What is claimed is:
 1. A foamable mixture in the form of a polymericconcentrate, said polymeric concentrate comprising a foamable polymer,an additive selected from the group consisting of platelet fillers,flame retardants, polar resin systems, and mixtures thereof, and ablowing agent for foaming said polymer composition and for causingmigration of said additive through the polymer composition, said blowingagent comprising a mixture of a polycarboxylic acid and sodium aluminumhydroxy carbonate, said polycarboxylic acid and said sodium aluminumhydroxy carbonate having been separately surface treated with a coatingmember selected from the group consisting of mono-glycerides, stearicacid, silane coupling agents, fatty acids, tatanates, oleates, andmixtures thereof, each of said surface treatments being carried out byseparately contacting said polycarboxylic acid and said sodium aluminumhydroxy carbonate with about 0.1 to 10 wt. % of said coating member at atemperature of about 72° C. to 135° C. under mixing conditions for asufficient period to form a coating on said polycarboxylic acid and saidsodium aluminum hydroxy carbonate which will avoid premature reactioncaused by moisture or water absorption, and blending the surface treatedpolycarboxylic acid and surface treated sodium aluminum hydroxycarbonate in a weight ratio of from 0.25:1 to about 10:1, respectivelyto form said blowing agent.
 2. A foamable product according to claim 1,wherein said polycarboxylic acid is selected from the group consistingof those of the formula HOOC--R--COOH and alkali metal salts thereof,wherein R is alkylene of 1 to 25 carbon atoms which may contain one ormore hydroxy substituents and may also contain unsaturation.
 3. Afoamable product according to claim 2, wherein said polycarboxylic acidis selected from the group consisting of citric acid, fumaric acid,tartaric acid, sodium hydrogen citrate, monosodium citrate and mixturesthereof.
 4. A foamable product according to claim 1, wherein saidadditive is a platelet filler selected from the group consisting ofmica, talc, clay, calcium aluminum hydroxide and mixtures thereof.
 5. Afoamable product according to claim 1, wherein said additive is a flameretardant selected from the group consisting of alumina trihydrate,magnesium hydroxide, magnesium carbonate, hydrotalcite and mixturesthereof.